healing factor ( or a little nosense)

Ok, bear with me: cullular births, in human bodies, continues to the age of 18 in males and 24-25 in females- (more or less) which is the age cellular degenration starts.. Now, if Logan and X-23 have an healing factor, it means that they should stop getting old at the age the reach full biologial and cellular maturity, right? So, if this si the case, why does Logan looks like a 40 years old man in the best of cases?